1. Field of the Invention
The present invention relates to flow measuring apparatus. More particularly, it relates to sensors for vortex flowmeters and to piezoresistive sensor assemblies.
2. Description of the Prior Art
It has been known for many years that vortices are developed in a fluid flowing past a non-streamlined obstruction. It also has been known that with certain arrangements that vortices are developed by alternately shedding at regular intervals from opposite edges of the obstruction to form corresponding rows of vortices. Such vortices establish a so-called von Karman "vortex street," which is a stable vortex formation consisting of two nearly-parallel rows of evenly-spaced vortices travelling with the flow stream.
In a von Karman vortex street, the vortices of one row are staggered relative to those of the other row by approximately one-half the distance between consecutive vortices in the same row. The spacing between successive vortices in each row is very nearly constant over a range of flow rates, so that the frequency of vortex formation is correspondingly proportional to the velocity of the fluid. Thus, by sensing the frequency of vortex formation it is possible to measure the fluid flow rate. Devices for that purpose are often referred to as vortex meters.
Various types of vortex meters have been available commercially for a number of years. Typically, these vortex meters comprise a vortex-shedding body mounted in a flow tube together with a sensor for detecting the frequency of vortex formation. Sensors used to detect the vortices often include diaphragms which fluctuate in response to alternating pressure variations generated by the vortices. For example, in U.S. Pat. No. 3,972,232 to Miller et al. and U.S. Pat. No. 4,220,046 to Sqourakes, pressure applied to the diaphragms is transferred to a piezoelectric sensor which then produces electrical signals responsive to differential pressure applied to the diaphragms. This differential pressure measurement is used, in turn, to measure the frequency of vortex formation and ultimately the fluid flow rate or velocity.
A limitation of this type of sensor is that it is capable of making only one measurement in a single process penetration, specifically, measuring differential pressure fluctuations used to compute the flow velocity. Additional instruments and process penetrations would be required in order to obtain additional measurement quantities, such as pressure or temperature. This increases the risk of releases of fugitive emissions and fluid loss and bears an increased cost for the purchase and installation of the additional instruments
Another disadvantage of additional process penetrations is the loss of accuracy in the measurements due to varying sampling points. Since the physical characteristics of the fluid changes within the flow, accurate measurements would require a common source point from which to sample within a single penetration.
It is the object of the present invention to provide a sensor which provides multimeasurement capabilities at a common source point within a single process penetration for use, in particular, within a vortex flowmeter.
A more specific object of the invention is to provide a more accurate computation of fluid density where the sensor is located in the shedder bar having a common sampling point directly within the process flow. A sensor located in this manner produces a more accurate computation of the amplitude of the shedding frequency which, in turn, produces a more accurate computation of the fluid density.
A further specific object of the invention is to employ the use of a sensor with piezoresistive materials which is capable of making multiple measurements.
A further specific object of the invention is to use the multiple measurements to derive other computations, such as absolute and kinematic viscosity, Reynold's No., and the mass flow rates of both liquids and gases.
A further specific object of the invention is to provide a replaceable multimeasurement sensor which is situated directly in the flow stream.
A further specific object of the invention is to provide a multimeasurement sensor situated within the flow stream having smooth contact surfaces for low fluid collectability for use in sanitary applications.
Other general and specific objects of this invention will be apparent and evident from the accompanying drawings and the following description.